Op-amp overdrive with two diodes each way in the feedback, 1987 style
Pedal and tube amp circuits, solved part by part the way SPICE (the circuit simulator engineers use) does it, while you listen and watch. Turn the knobs, change any part, or tap a wire to put a probe on it.
How to use the bench
Every part is simulated. Nothing you hear is a recording.
1Pick a circuit
Choose one from the menu, or type a name, maker or year in Find a circuit. The buttons under the menu switch between versions.
2Set the knobs and the input
Turn the circuit’s knobs on the front panel. In the Input box, choose Guitar or Signal generator, and the note or chord.
3Press Play
Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.
4Tap a part
Tap any part on the schematic to change its value, or swap the transistor, tube or diode. The red numbers are DC voltages, and they update as you go.
5Tap a wire
Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.
6Try a change
Press Try it on a card under Try this to hear one change, before and after. Modify this circuit, below the schematic, adds or removes parts.
Good to know
- Not sure what a control does? Hover over it, or hold a finger on it, for a short note.
- For step-by-step walkthroughs, open Start here at the top of the page.
- To put together a pedal or preamp of your own, use the Builder tab.
- Your switches stay where you left them the next time you open that circuit, in this browser only.
- A tube amp opens whole, with its power amp. Pick Preamp only in the Power amp menu to hear the preamp alone, or another power amp to try this preamp on it.
- The graphs use a steady test tone. Play sounds a picked note that dies away.
- Part values are typical of their type. Each circuit’s notes say which schematic it’s based on.
Modeled after the Yamaha OD-10MII.
An op-amp buffer, an op-amp gain stage with two silicon diodes in series each way across its feedback, a passive tone control and Level, and a transistor output buffer.
Try this
Common mods
Changes people often make to this circuit. Try it plays the note before and after, at the same loudness. Keep it adds the mod to Your changes, where it can go on a switch.
Input
Peak voltage from the pickup for each note. Strum hard and a single coil puts out a few hundred millivolts. When you play a chord, its strings share this level.
Your build
Stack blocks from the guitar down to the output. Tap + Add a block wherever you want one. The coupling caps and the supply are wired in for you.
Your blocks. These are blocks you saved with Save block, values and part changes included. Add one to any build of the same kind with + Add a block. They stay in this browser.
Your builds stay in this browser, on this device, and nothing is sent to us. Anyone using this browser can see them. They’re lost if you clear your browsing data or use a private window, so Save to a file to keep a copy. Open a file, or drop the .bench file on the page, brings it back on any device.
Modify this circuit Add parts, take them out, or put one in series with another, then hear what it does. Tap to open.
Pick what to do, then tap the schematic. Your changes show in purple. Any change can go on a switch beside the knobs, from the menu in its row under Your changes.
Tap where each leg goes. Tap both ends of a part to put the new one across that part.
With Remove a part on, tap any part of the pedal or amp to take it out, and tap it again to put it back. Tapping a purple part you added deletes it.
Turn on Add in series with a leg and tap a part. In that part’s box, below the schematic, pick which leg to lift and what goes in the gap, such as a resistor between a cap and ground or a stopper in front of a grid.
With Cut into a wire on, tap any wire. Pick a part from the menu and it goes in the gap, wired in. Volume pot puts a pot there with the input on one end, the output on the wiper and the other end to ground.
With Run a wire on, tap a leg, then where the wire goes. Drag any piece of a wire you ran, or of an added part’s lead, to square it up or move it.
Your changes stay in this browser, on this device, for your next visit. Nothing is sent to us. Anyone using this browser can see them. They’re lost if you clear your browsing data or use a private window, so save a file to keep a copy. A shared link carries the circuit after the #, a part browsers never send to a server.
Input
pickup, unloadedat the pedal input
Output
at the amp input
Probe
Spectrum
Frequency response
What the curves mean
The ink curve is the response to a very quiet signal, from the generator to the probe. The blue curve is measured with pink noise at the level you set, like an audio analyzer. When stages clip, the blue curve pulls away from the ink one and the gray coherence line (right-hand scale) drops below 1. Then no single curve tells the whole story. Both use the signal generator, whichever Input tab is picked.
Output
Bias
every voltage and current at rest
What to listen for. IC1B’s gain is 1 plus R4 and the Overdrive pot over 220 Ω, from about 56 to over 500, and C4 cuts it below about 720 Hz. With two diodes in series each way the output swings to about 1.2 V either way before they conduct, so it clips softer and louder than one diode each way. Tone fades between R6 and R7, a flat half of the signal, and R8 with C6, a low-pass near 190 Hz. Turned up it gets brighter. The model leaves out the bypass switching.
Download.
Parts and what they do
Stock values. Tap a part on the schematic to change it.
- R13, 22 kΩ
- Top of the 4.5 V reference divider.
- R14, 22 kΩ
- Bottom of the reference divider.
- C9, 47 µF electrolytic
- Filters the reference.
- C1, 47 nF
- Input coupling cap.
- R1, 10 kΩ
- Input series resistor.
- R2, 1 MΩ
- Biases the input buffer and sets the input impedance near 1 MΩ.
- IC1A, RC4558
- Input buffer, a follower. An RC4559 in the pedal. The RC4558 stands in.
- C2, 47 nF
- Coupling cap into the gain stage. Against R3 it cuts below about 34 Hz.
- R3, 100 kΩ
- Biases IC1B’s plus input at 4.5 V.
- IC1B, RC4558
- Gain stage, non-inverting, with diodes across its feedback.
- C3, 560 pF
- Across the feedback. With Overdrive up it rolls the top off from a few kHz.
- D2 to D5, 2 × 1N914 / 1N4148, one each way
- Four 1N914s across the feedback, two in series each way. Past about 1.2 V either way they pull the gain down.
- R4, 12 kΩ
- Least feedback resistance, so the gain never drops below about 56.
- Overdrive, 100 kΩ pot, audio taper
- Overdrive, a 100 kΩ audio pot wired as a variable resistor in the feedback.
- R5, 220 Ω
- The gain leg. Gain is 1 + (R4 + Overdrive) / 220 Ω.
- C4, 1 µF
- In series with R5, so the gain falls below about 720 Hz.
- C5, 1 µF
- Coupling cap to the tone network.
- R6, 18 kΩ
- With R7, the flat side of the tone network, half the signal.
- R7, 18 kΩ
- Bottom of the flat side.
- R8, 18 kΩ
- With C6, the dark side of the tone network, a low-pass near 190 Hz.
- C6, 47 nF
- The low-pass cap.
- Tone, 100 kΩ pot, audio taper
- Tone, 100 kΩ audio, fading from the dark side to the flat side. Turned up it gets brighter.
- Level, 100 kΩ pot, linear taper
- Level, 100 kΩ linear, on the tone wiper.
- C7, 1 µF
- Coupling cap into the output buffer.
- R9, 220 kΩ
- Biases the output buffer at 4.5 V.
- Q1, 2N5088
- Output buffer, an emitter follower.
- R10, 10 kΩ
- Output buffer emitter resistor.
- R11, 470 Ω
- Output series resistor.
- C8, 1 µF
- Output coupling cap.
- R12, 100 kΩ
- Output pulldown.
About this version
- OD-10MII
- Values from Aion FX’s clone. No factory drawing was at hand. An RC4558 stands in for the RC4559.
How this tool works. Every part is an equation, and the whole circuit is solved several times per audio sample: 192,000 samples a second while you drag a knob, 384,000 once you let go. Transistors use Ebers-Moll with gain, leakage, Early effect, base resistance, junction capacitance and temperature. Their charge storage is set at the bias point, close for small signals and rough while they switch hard. Tubes use Koren’s equations fitted to each datasheet, plus grid current. Diodes are exponential junctions with series resistance. Presets are typical of each type. Caps are ideal, and the battery or HT is a voltage behind a resistance.
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